While the broadband output of a free running excimer laser is adequate for many applications, for certain applications it is necessary to spectrally narrow the laser output. Two examples are remote sensing ("Excimer Laser System for Atmospheric Remote Sensing of Ozone" by K. O. Tan et al., paper presented to Lasers '86, Orlando, Fla., Nov. 3-7, 1986, as noted on the first page thereof), and excimer photolithography ("Excimer laser based lithography: a deep-ultraviolet wafer stepper for VLS1 processing" by V. Pol et al., Optical Engineering, April 1987, Vol 26, No 4, pp 311-318). In the latter case the need arises due to the difficulty of fabricating achromatic (i.e. colour corrected) lenses that operate in the deep ultraviolet. For example, Pol et al point out that the normal spectral width of a single stage KrF excimer laser is approximately 0.7 nm FWHM, whereas their lens design required a bandwidth of 0.005 nm FWHM (Full Width at Half-Maximum, i.e. the accepted way of measuring or denoting the width of a spectral peak, laser pulse, or the like).
There are several ways of line narrowing an excimer laser. These are summarized in "Spectral-narrowing techniques for excimer laser oscillators" by T. J. McKee, Canadian Journal of Physics, Vol 63, 1983, in particular in FIG. 1. These methods have become well established for all major excimer transitions, as noted in Table 2 of this McKee paper.
In certain applications, it is not sufficient to merely narrow the bandwidth of an excimer laser. A method to maintain the output fixed at the chosen narrow wavelength is also required, i.e. to constantly tune the laser. This can be done through the use of a feedback loop.